Elongated guiding members are widely used in medical procedures. A common example are the guidewires used to deliver intravascular devices such as angioplasty catheters. Since guidewires must traverse the peripheral and tortuous coronary vasculature in order to reach the desired treatment location, they must exhibit a number of important characteristics. Specifically, a guidewire should have sufficient strength and elasticity to impart suitable pushability, trackability, torqueability, flexibility and handleability. A major requirement for guidewires and other guiding members, whether they be solid wire or tubular members, is that they have sufficient column strength to be pushed through a patient's tortuous vascular system or other body lumen without kinking. However, they must also be flexible enough to avoid damaging the blood vessel or other body lumen through which they are advanced. Efforts have been made to improve both the strength and flexibility of guidewires to make them more suitable for their intended uses, but these two properties are for the most part diametrically opposed to one another in that an increase in one usually involves a decrease in the other.
Prior art guidewire designs have made use of shape memory and/or superelastic alloys such as nickel-titanium (“Nitinol”). The shape memory characteristics allow the devices to be deformed to facilitate their insertion into a body lumen or cavity and then be heated within the body so that the device returns to its original shape. Superelastic characteristics on the other hand generally allow the metal to be deformed and restrained in the deformed condition to facilitate the insertion of the medical device containing the metal into a patient's body, with such deformation causing the phase transformation from austenite to martensite. Once within the body lumen the restraint on the superelastic member can be removed, thereby reducing the stress therein so that the superelastic member can return to its original undeformed shape by transformation back to the original austenite phase.
Despite the advantages offered by the use of superelastic and shape memory alloys, prior art designs have often suffered from insufficient performance characteristics. For example, U.S. Pat. No. 4,925,445 (Sakamoto et al.) discloses the use of a nickel-titanium superelastic alloy in an intravascular guidewire which could be processed to develop relatively high yield strength levels. However, at the relatively high stress levels which cause the austenite-to-martensite phase transformation characteristic of the material, the material of Sakamoto et al. did not have a very extensive stress-induced strain range in which the austenite transforms to martensite at relative constant stress. As a result, frequently as the guidewire was being advanced through a patient's tortuous vascular system, it could be inadvertently stressed beyond the superelastic region, i.e. develop a permanent set or even kink which can result in tissue damage. This permanent deformation would generally require the removal of the guidewire and the replacement thereof with another. Alternatively, U.S. Pat. No. 4,665,905 (Jervis) teaches alloys having extensive strain ranges, but the relatively constant stress level at which the austenite phase transformed to martensite was very low.
Accordingly, there remains a need for guidewires formed of superelastic alloy material having superior performance characteristics. Specifically, there is a need for such guidewires that have sufficient stiffness to exhibit good pushability coupled with superelastic properties that maximize flexibility and minimize kinking. This invention satisfies these and other needs.